2. The size power supply depends on the graphics card. They can run as high as 450 Watts. But the stock power supply is usually only 250 to 300 Watts. My guess is that you most likely will need a better power supply.

3. A slimline box is not the best option to build a gaming PC. There is not a lot of room and it may already be difficult to find even a suitable power supply. And some graphics cards need a lot of room too. Make sure you check all the measurements before you go any further.

djhull, as whs said, some graphic cards require a lot of power to run. If your power supply is 250-300watts (which usually is the case with the kind of computer you posted), you will definitely need to get a bigger / better power supply. Or you can buy a cheaper graphic card to stay within the limits of your current power supply.